This invention relates to a time-delay-triggered TRAPATT oscillator arrangement comprising a TRAPATT diode, a length of delay line connected across the diode and a trigger element in the form of a step transition from a high to a low impedance at the end of the delay line remote from the diode. Such an arrangement is described by W. J. Evans in "Circuits for high-efficiency avalanche-diode oscillators", I.E.E.E. Trans. MIT-17, 1060, (1969). However, it has been reported (J. E. Carroll, "The use of pseudo-transients in the solution of the Evans TRAPATT circuit", Proceedings of the 8th International MOGA Conference, Amsterdam, 1970; J. E. Carroll and R. H. Crede, "A computer simulation of TRAPATT circuits", Int. J. Electron. 32, 273 (1972)) that successful operation of this circuit depends markedly on a number of parameters, and the complexity of the circuit (which comprises series tuners in the form of transmission lines of fairly critical lengths and impedances) is a considerable hindrance to the design and construction of simple TRAPATT oscillator modules. In particular, since the Evans circuit relies for its operation upon repeated triggering of the diode by pulses reflected from the junction of the delay line and the filter, then reflections from within the filter itself or from circuit elements beyond the filter can cause unwanted, spurious triggering. As can be appreciated from FIG. 6 of the Evans paper such reflections from within the filter can be caused by large impedance mismatches between the successive portions of the filter which, in the arrangement described with reference to FIGS. 4 to 6 of the Evans paper is in the form of a coaxial line with tuning sleeves.
Thus, this conventional TRAPATT oscillator circuit suffers from the following disadvantages.
Since each transition from a high impedance to a low impedance transmission line (as one moves away from the TRAPATT diode) is capable of generating a trigger voltage from a single diode-stimulus, multiple triggering is possible. Since each of these multiple trigger pulses can compete to control the oscillation frequency, coherence requires one trigger to achieve dominance.
Little, if any, serious investigation appears to have been carried out on the practical problem of preventing the diode voltage from exceeding avalanche breakdown between trigger pulses, particularly during the transient phase of the TRAPATT mode, that is to say during the time period between the first TRAPATT pulse and the cycle in which coherence is established. Increased lumped local capacitance and a trigger-line with single impedance stop have been found to be advantageous from experience but it is likely that such voltage suppression has, in the main, been inadvertently achieved by reflections from discontinuities between low impedance and high impedance transmission lines from components between the TRAPATT diode and the radio-frequency load.
Hitherto it has been thought that TRAPATT oscillator circuits merely provided the necessary steady state impedances at the fundamental and harmonically related frequencies, but in practice it is thought that it also provided the voltage-steps for suppressing the diode voltage between trigger pulses. Thus, to achieve coherence to accommodate this unanticipated role the many elements of the Evans circuit almost always require empirical adjustment. Failure to prevent the diode voltage exceeding avalanche breakdown between triggers will lead to different diode states prior to each trigger pulse and hence preclude coherence.